The standard thermostat operates by providing either an "on" signal or an "off" signal to the heating and cooling system, which causes the heating or cooling system to either run at full speed or be in an off state. Traditional HVAC equipment with compressors may have two speeds: minimum and maximum, giving the system limited variable capacity. The thermostats which control this equipment provide an on/off signal for each speed. Newer HVAC equipment may have variable speed compressors which are capable of a minimum "on" speed and are variable up to a maximum speed.
If the HVAC system which is utilized is a gas engine heat pump, the gas engine heat pump allows for several states. The gas engine heat pump can be off, on at an idle speed, or on at some greater value than idle, up to and including a maximum speed. Gas engine heat pumps are a recent development and are not currently available to the general public.
For a thermostat to operate with a gas engine heat pump, that thermostat is required to provide a signal which indicates the load (the percentage of the capacity of the HVAC equipment required to maintain the set point temperature). In this way, the gas engine heat pump is able to run at a higher speed as the load increases. As a result, a gas engine heat pump is better able to match the needs of the system.
The second condition which must be accounted for in a gas engine heat pump is the condition when setback thermostats are utilized. A setback thermostat will change the desired temperature at a set time to either increase or decrease the desired temperature with the intention of saving energy. For example, a standard setback thermostat, when operating in a northern climate in the winter, may increase the desired temperature in the morning when the occupants of the house are home. The thermostat will then decrease the desired temperature during the day when the occupants are away at work or school, and increase the temperature when the occupants return home. Finally, the thermostat will decrease the temperature when the occupants go to sleep. A gas engine heat pump operating in such an environment may be running at a high speed prior to the setback thermostat setting the temperature to a lower temperature, thus the signal from the thermostat would generally shut the gas engine heat pump off. Such a drastic change in speed will shorten the life of the gas engine heat pump. A second condition is where the thermostat would increase the desired temperature. In such condition, the gas engine heat pump would go from an off state to a maximum on state in a short period of time. Similarly, this would also shorten the life of the gas engine heat pump.